Blood Pressure Part One: What is “normal?”9 minute read

Steven Kornweiss, MD cardiovascular Leave a Comment

Summary First

If we want to live the longest and healthiest lives possible, we’ve got to delay cardiovascular disease, cerebrovascular disease, and kidney disease. It’s well known that people with sub-“normal“ blood pressures have a better chance of avoiding or delaying these diseases. I’m interested in the idea that achieving and maintaining lower than “normal” blood pressures should be part of a broader longevity strategy. This first article on blood pressure provides some background on what blood pressure is, why it matters, where “normal” comes from, and sets the stage for my argument that lower is better, which I’ll expand upon in part two.

Hot Water Heater

A single bead of water formed on the outside of the pipe. I blotted it dry. It reappeared. I dried it again, but once more, the drop reappeared. Condensation, I thought. But then I realized it couldn’t be. Water condenses out of relatively warm and humid air onto a colder surface. But this wasn’t cold, it was the hot outlet pipe from my water heater. Even though I couldn’t see it, there had to be a hole. I called a plumber who confirmed my diagnosis and replaced the pipe.

This leaky pipe story demonstrates the long-term effects of trying to contain fluids that are under high pressure within narrow conduits. Loosely speaking, this is the same problem we face with a lifetime of trying to convey blood from the heart and lungs to our organs via our arteries. Over time, these vital tubes (our arteries) can become obstructed, narrowed, can stretch out, can leak, or rupture. What’s more, the heart, which must propel the blood through our arteries for decades, can become prematurely diseased if it’s forced to pump against an overly high-pressure system.

What is Blood Pressure?

Blood pressure is the pressure exerted by our blood on the walls of our arteries. It’s traditionally measured using a cuff on the arm called a sphygmomanometer—literally, a pulse pressure meter.

The 1936 Pulsomètre du Prof Frossard | Image Courtesy of Uwe Diegel

The blood pressure cuff is wrapped around your arm and is inflated to high pressure until the arteries in your arm are occluded—like pinching a hose. The doctor or nurse then gradually alleviates pressure by deflating the cuff. As the pressure in the cuff declines, we listen for the return of blood flow. Just as we hear the blood flow return, we record the pressure. At this moment, the pressure generated by the pumping of the heart is just enough to push blood past the partially inflated cuff. This is the systolic pressure—the highest pressure in the arteries as the heart pushes against the blood to propel it forward.

For many decades, the best sphygmomanometers used mercury (Hg) pressure gauges. Even though this is no longer the case, blood pressure is still measured in millimeters (mm) of mercury. A “normal” systolic blood pressure is 120 mmHg or less.

To measure the diastolic pressure, we continue to release pressure until the flow of blood is just barely audible. At this moment, the artery is now nearly unobstructed by the cuff and blood can pass freely underneath its surface. At this moment, we record the pressure. This is the diastolic pressure—the pressure of the blood against the artery wall when the left ventricle is in diastole—that is, when the heart is in between beats and is not pushing the blood forward. This is the diastolic blood pressure. A “normal” diastolic blood pressure is 80 mmHg or less.

A Brief Discussion of the Circulatory System Anatomy and Physiology

The Circulatory System is a Closed Loop

Let me back up just a little bit. In order to really understand why blood pressure matters, it helps to know a bit about the circulatory system. The circulatory system is a closed loop. Blood circulates from the heart through a series of vessels which branch into ever smaller caliber tubes as the blood travels further from the heart. Once the blood reaches an organ like the liver or pancreas, or a muscle, the vessels have become small enough and thin enough that individual cells can access the oxygen and nutrients within the blood. These narrow and thin-walled vessels are called capillaries. The capillary bed eventually coalesces back into bigger caliber vessels called veins through which blood then returns to the heart and lungs. It’s a closed loop, so there’s no real start or end, but we can think of the left side of the heart as the starting point.

The left side of the heart?

Yes, the heart has two sides. Right and left. The heart is really a complex of two pumps that are joined together and move as one unit, with the pressures and movements of one affecting the other.

Each side has two chambers, an atrium and a ventricle. The atria are named as such because they are the sort of receiving rooms for blood before it’s transferred to the ventricles. In Latin, atrium means an open-roofed entrance hall or central court.

The ventricle is the strong muscular chamber that does most of the pumping. The heart is really two separate pumps. And, in fact, you might even think of the heart as a sort of bellows, which both sucks blood into itself as it expands and pumps blood out as it contracts.

Courtesy of: DrJanaOfficial – Official Website, Support, CC BY-SA 4.0

A Schematic of the Circulatory System

The right side of the heart pulls blood back from the vital organs which have just used it. The right ventricle pumps blood into the lungs which allow exhaust to evaporate out of the blood (CO2) and which also allow oxygen to dissolve into the blood. The left heart pulls this fresh blood from the lungs and then pumps it forward to every vital organ in the body, including itself.

The circulatory system has much in common with the plumbing in your house.

Fresh water comes in from a reservoir, the city, or perhaps from a well. This is like the fresh oxygenated blood in the left side of the heart. This fresh water is under pressure as it comes into your house. The pipes that carry the water to the faucets are like the arteries of the house. They are thick strong pipes made of copper or hard plastics. When someone opens a faucet, water flows out under high pressure. The arteries are the same way. They are thick, muscular tubes that convey blood under high pressure to our organs (e.g. brain, heart, liver, muscles, etc.).

Water flows out of your faucet and then down the drain, into the drainpipes, and ultimately into the sewer or the septic system. The drainpipes are like the veins of the house. Water flows down these pipes driven only by gravity. There is no additional force behind it, so the drainpipes have very little pressure inside of them, just like your veins.

Our concern is with the pressure within the high pressure pipes—arterial blood pressure.

When we measure blood pressure, this is what we’re measuring, the pressure within the arteries, not within the veins. Arterial blood pressures are usually something like 120/80 mmHg whereas venous pressures are more like 5-10 mmHg. Amazingly, some people can walk around with blood pressures of 180/100 for many years. On the opposite end of the spectrum, some people have natural blood pressures around 90/50.

The arteries, being the higher pressure system of the two, are more complex than veins. Arteries have thick muscular walls that can contract and relax to divert blood flow and to accommodate changes in blood volume and cardiac output. For example, after we eat, the arteries that supply blood to the digestive tract dilate to increase the blood supply. Similar effects take place in the arteries of the arms and legs during exercise.

Because the arteries are a high pressure dynamic system, they’re more prone to catastrophic complications such as occlusion, dissection, and rupture.

When this happens, blood flow into the organ supplied by the damaged artery can be compromised. When blood flow ceases through an important artery, the organ in question can suffer ischemia or infarct. If an artery ruptures, hemorrhage can occur in the surrounding area. In other words, when an artery fails, the organ it supplies can die due to lack of blood flow or it can be destroyed by internal bleeding.

What is “Normal” Blood Pressure?

By the guidelines:

  • Normal blood pressure is considered to be below 120 systolic and less than 80 diastolic.
  • Stage 1 hypertension is defined as 120-139 / 80-89.
  • Stage 2 hypertension is defined as greater than 140 systolic or 90 diastolic.

Here is a table view of the categories of blood pressure from the most recent American College of Cardiology guidelines on blood pressure:

How do we know that 120/80 is normal?

According to the guidelines:

“The rationale for this categorization is based on observational data related to the association between SBP/DBP and CVD risk, RCTs of lifestyle modification to lower BP, and RCTs of treatment with antihypertensive medication to prevent CVD. The increased risk of CVD among adults with stage 2 hypertension is well established. An increasing number of individual studies and meta-analyses of observational data have reported a gradient of progressively higher CVD risk going from normal BP to elevated BP and stage 1 hypertension.”

What they’re saying is that the threshold for “normal” is defined with respect to disease rather than what’s normal for a healthy individual.

Approaching Blood Pressure From the Standpoint of Longevity

If we’re approaching blood pressure from the standpoint of longevity, then we can no longer be satisfied with what’s “normal” per guidelines. These guidelines are for general public health. They’re not designed for those looking to live as long as possible in the best possible health. So, instead of talking about “normal,” let’s talk instead about the healthiest blood pressures.

What is The Healthiest Blood Pressure?

If we look at blood pressures in a population of young adults, studies reveal that it’s roughly 100-115 / 65-75. Some people may even be a little lower than this. It seems true that even the healthiest people end up having increases in blood pressure as they age, perhaps 2 or 3 mmHg for every decade of life starting in the 5th and 6th decades of life.

So-called “super-agers,” centenarians in good health, have average blood pressures in the range of 130-140s/70-90 towards the end of their lives. It’s thought that systolic blood pressure rises with age (even in healthy adults) while there is a relatively lesser increase in diastolic pressure. The result is a wider pulse pressure (difference between systolic and diastolic blood pressure). This is likely due to a decrease in the elasticity of the arteries that occurs with aging.

Of course, there are people in excellent health with blood pressures that are both above and below these ranges, but it’s reasonable to consider 100-115/65-75 mmHg to be healthy pressures for young adults and 130-140/70-90 mmHg as healthy for older adults. These ranges would put you roughly in the bottom quartile of the population.

The ranges I’ve just mentioned above are healthy, but there’s evidence that individuals with blood pressures even below 100 systolic might be at lower risk for cardiovascular disease than those between 100 and 115. In actuarial studies, mortality is lowest for those with blood pressures around 90s/50s. Furthermore, the effects of blood pressure on the cardiovascular system are likely cumulative over one’s lifetime such that even tiny increases spread across a long period of time may be damaging.

Like everything else in medicine, blood pressure is a parameter that varies from person to person. What’s more, it varies throughout the day and over time. An excellent starting place to determine what’s best for an individual might come from their own medical history. We might go back to the earliest post-adolescent blood pressure we can find. If a person was in excellent health at the age of eighteen or nineteen, and their blood pressure from that time falls into this 90-115/50-70 range, that would indicate a good starting point from which to estimate a person’s healthiest blood pressure.

If an individual was never in excellent health after adolescence, it might be hard to know what that person’s blood pressure could or should be. In that case, more detailed analysis and experimentation might be necessary.

Overall, determining the healthiest blood pressure for an individual is probably a matter of finding the lowest possible blood pressure that a person can achieve within their own physiologic limitations.

Part Two

I’m going to stop there for now. In part two, you can expect to read about the following:

Based on a comprehensive review of the medical literature, discussion with clinical cardiologists, my own clinical experience, and careful thought, I’m going to present the idea in part two of this series that maintaining blood pressure below what’s considered “normal” should be part of a broader strategy to delay the onset of cardiovascular and cerebrovascular disease.

If you want to be sure you get part two of this in your inbox, I recommend subscribing to my free newsletter.

Best,

Steve


Featured Image: Vaquez Laubry by Spengler | Image Courtesy of Uwe Diegel


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